Really? It's happened to me on at least 3 occasions. Each time the discussion didn't happen until less than 24 hours before the gig.
...I think word has gotten around Knoxville that I'm the only college bass player with a rig large enough to handle big venues
In this case young man, you need to grow some b*****s! If anyone shows up and doesn't bring their own rig, then they aren't experienced, skilled or professional enough to use any gear, let alone yours. They can go direct through the PA and take whatever pathetic monitor feed they can get.
You found a way to pay for your stuff, other guys should pay for theirs too. If that happened to you three times, then you better just say no. You said it was a "discussion" but it seems more like it was "hey, we are using your rig." In future cases I would tell everyone no. I would ask the venue/promoter, if its his idea, to guarantee you in writing that if the other guys damage your stuff he pays cash up front that night to repair or replace. If he doesn't have cash in hand before the night starts, then you pull your stuff until only you are playing.
If its the other bands idea, you charge him rent. You ask him to bring cash, say $300. If he makes it through the set without killing your stuff, you give him back $280. If he damages it, you keep the $300 so you can fix or replace it.
If you do this I'm sure you won't ever have any other problems or non-negotiated last minute backline requests.
And no, in 30 years I've never let my rig be used for backline without much conversation or some financial guarantee/rent. I learned quickly, since I also booked a few shows independently, that as a club/promoter for original music you would have to rent sound gear for some clubs and if you didn't have the ability to load in each band's gear you have to provide the backline yourself (its called a rental.) If they are basically forcing you to provide the backline then you are providing him/her a much cheaper/free alternative. That is a no. I'm sure they don't guarantee you guys a set amount, only a piece of the door, which often is zilch or near zilch.
My Hartke gear lately gets more use for guys throwing benefit shows then me playing it. I will allow for its use, often rent free, but get a credit card from the organizer in case there is other then normal bar/show wear. They understand the deal. I donate my gear, but I'm not donating it to be damaged. Since its only a security, they never have problems.
I had one of my old Ampeg 1X15 cabs killed at a benefit last fall, one of the "volunteer roadies" was a little soused at the end of the show and managed to spear the cab with a perfectly sharp microphone stand as he tried to imitate Steven Tyler whipping the mic stand around. Perfect bullseye. The organizers of the benefit were not around for me to talk to, so I had the CC charged immediately for $300. As you guys know that much barely buys you a new, equivalent cab and might barely pay for a speaker replacement without a new grill.
A few weeks later I got a call from them asking about the charge and I referred them to the call and email I made to them. They were a little miffed at first because they had distributed all the funds and wanted me to eat the cost of repair/replacement and now they had a personal charge and no funds left to pay for it. I didn't budge.
Neither should you young man. The cost of someone damaging your "large venue rig" is more then just the invoiced cost. You lose use of it, your time, your gas, your mileage, and its always your money upfront. And as we all know, if others don't have a rig like yours then most likely they either can't afford to fix/repair/replace it nor would they actually come up with the cash for you.